1. Field of the Invention
The present invention relates to a technique for forming a resonance part using an inductance element and a variable capacitance element, and a voltage controlled oscillator (VCO) and an electronic component using the resonance part.
2. Description of the Related Art
As shown in FIG. 19, for instance, a voltage controlled oscillator (VCO) includes a resonance part having a varicap diode VD whose electrostatic capacitance varies in accordance with a control voltage and an inductance element 11, and a feedback part 2 formed of a transistor 21 being an amplifying part and two capacitors C1, C2. In this example, a frequency signal resonated in the resonance part is amplified by the transistor 21, and is fed back to a series resonant circuit via the feedback part 2, thereby forming an oscillation loop. Note that 31 in FIG. 19 denotes a buffer amplifier that amplifies the frequency signal and outputs it to the outside. Further, 16, T3 and L denote an input terminal, an output terminal part and an inductance element, respectively. Although the illustration is omitted, the VCO is disposed on a base substrate made of ceramics such as LTCC (Low Temperature Co-fired Ceramics) using alumina (Al2O3) as a main component, for instance.
Incidentally, if the VCO is used in a high frequency band of, for example, several GHz or several tens of GHz by increasing an oscillation frequency thereof, problems as follows occur. Specifically, in a high frequency band, an absolute frequency included in a bandwidth of a peak part in a frequency characteristic is large, so that it becomes necessary to provide a Q value which is higher than that in a low frequency band. In order to achieve such a high Q value, the inductance element in the resonance part is required to have a large inductance value with a low loss.
Here, when a strip line formed on a base substrate is used as the inductance element, since a dielectric loss tangent (tan δ) of LTCC is large, a good characteristic regarding the Q value of the resonance part being a reciprocal of the dielectric loss tangent cannot be achieved. Further, when fluorocarbon resin is used as the base substrate, since a dielectric loss tangent of the fluorocarbon resin is about 0.001, the Q value of only about 1000 can be obtained. For this reason, a good phase noise characteristic cannot be obtained, and it is difficult to reduce power consumption. Further, when the base substrate is formed of a low-loss dielectric, although a high Q value with a low loss can be obtained, a size of the substrate becomes large.
Furthermore, in a VCO mounted in a radio communication device that outputs an electrical signal with a quite high frequency, which is, for example, an electrical signal in a GHz band (microwave), it is conceivable that a) an output signal of the VCO outputting a signal having a frequency lower than the frequency band is multiplied, b) GaAs (gallium arsenide compound) or the like is used as a base substrate, and c) a cavity resonator is used. However, in the case of a), a phase noise becomes large since a multiplication circuit is used. Further, in the case of b), a phase noise in the vicinity of an oscillation frequency of the VCO is deteriorated, and in the case of c), it becomes difficult to realize the downsizing.
Although Patent Document 1 describes a device such as the above-described VCO, no study has been made regarding the aforementioned problems. Further, according to Patent Document 2, there is known a technique for making a quartz crystal being a piezoelectric substrate generate an elastic wave to use the quartz crystal as a resonator whose oscillation frequency is in about several MHz band, for example, but, it is not possible to solve the above-described problems.    [Patent Document 1] Japanese Patent Application Laid-open No. Hei 10-209714    [Patent Document 2] Japanese Patent Application Laid-open No. 2007-201772